Flyash is a waste material, normally recovered by coal-burning furnaces. It is composed mostly of metal oxides, such as silica, alumina, iron oxide, and magnesium oxide, together with unburned carbon resulting from the combustion of coal. Flyash is believed to be one of many pozzolanic materials, such as volcanic ashes and diatomaceous earths, which react with lime (CaO) in the presence of water to produce a hard cementitious compound at ambient temperatures. In fact, certain classes of flyash, which are the by-product of western lignite coal power plants, are known to contain up to about 30% CaO. This type of flyash has been designated as a Class C artificial pozzolan as recognized by the ASTM C-618 standard. There is also a Class F flyash, which contains less than about 6% CaO, and hence, is rather inert, with little or no binding ability.
Flyash has been employed in such diverse applications as in the stabilization of sludge, U.S. Pat. No. 4,615,809, in the improvement of the flow and strength characteristics of concrete, see U.S. Pat. Nos. 4,240,952 and 4,318,744, and more recently in the preparation of cementitious floor underlayments, see U.S. Pat. No. 4,661,159, all of which are hereby incorporated by reference. As these patents illustrate, flyash has been most often used as a minor ingredient in cement mixtures due to its low green strength and extremely quick-setting ability. As a consequence, flyash has been lagging behind cement in the number of commercial applications, and the properties and chemistry of flyash have not been completely understood.
Unfortunately, the commercial leader of binders, cement, has not been completely satisfactory. Cement tends to shrink during setting, and, after weathering, often develops large gaping cracks. Cement must also contain large quantities of aggregates, such as gravel, when disposed in thicknesses greater than about 1 inch. Finally, cement-based binders require about 24 hours to set; a factor that most contractors anticipate, but which has nevertheless resulted in significant and costly completion delays.